There is great demand for structural materials that can withstand adverse environments at increasingly higher temperatures and stresses. The upper operating temperature of conventional heat resistant alloys is limited to the temperature at which second phase particles of the alloy are substantially dissolved in the alloy matrix or to the temperature at which the particles become severely coarsened. Above this limiting temperature the alloys no longer exhibit useful strength.
The high melting point, low density and excellent oxidation/corrosion resistance of binary NiAl make it an excellent candidate for high temperature structural purposes. Its poor mechanical properties at elevated temperature, however, indicate that use of the monolithic material is improbable. In U.S. Pat. No. 4,961,905 to Law et al., NiAl intermetallic materials are modified by adding amounts of an alloying material that render the NiAl structure susceptible to a martensitic transformation. The preferred alloying element is cobalt, which can double or triple the fracture toughness of NiAl materials while simultaneously increasing the room temperature yield strength. Another approach for improving the properties of NiAl is by incorporating continuous reinforcing fibers for increased strength and improved damage tolerance. Single crystal Al.sub.2 O.sub.3 is the reinforcing fiber of choice because of its high modulus and strength, low density and chemical stability.
The mechanical properties of polycrystalline NiAl reinforced with 30 vol. % continuous single crystalline Al.sub.2 O.sub.3 fibers were investigated and reported by Bowman, in "Influence of interfacial characteristics on the mechanical properties of continuous fiber reinforced NiAl composites," Mat. Res. Soc. Symp. Proc., Vol. 273, 1992. Bowman reports the mechanical properties of a NiAl composite reinforced with either weakly or strongly bonded Al.sub.2 O.sub.3 fibers. A strong interfacial bond between the Al.sub.2 O.sub.3 and the NiAl is a requirement for oxidation resistance. However, the difference between the coefficients of thermal expansion for the Al.sub.2 O.sub.3 fibers and for NiAl, together with the interfacial strong bond, contributes to severe crack formation in the composite due to thermally generated internal stress. In the powder-cloth ("P-C") process for composite production discussed in Bowman, alloy matrix material is processed into flexible cloth-like sheets by combining matrix powders with an organic polymer binder. The presence of the polymer binder inhibits fiber-matrix bonding, thereby increasing the matrix's resistance to cracking. However, in addition to low oxidation resistance afforded the material due to the weak bond, fiber distribution can be irregular and difficult to control with P-C processing. This problem is considered by Bowman et al., in "Unresolved technical barriers to the development of a structurally viable Al.sub.2 O.sub.3 /NiAl composite," HITEMP, NASA-CP-19117, 1993. Thus, efforts to increase the strength of NiAl by alloying and compositing with continuous Al.sub.2 O.sub.3 fibers have shown only limited improvements in the properties required for high temperature structural applications.
Mechanical alloying and related processes produce alloy materials with novel microstructures which can be used in structural applications. Cryomilling is high energy milling at cryogenic temperatures. Depending on the material system being used, cryomilling can also be a reaction milling process as in the case of milling prealloyed NiAl in liquid nitrogen. As the NiAl is fractured in liquid nitrogen, it reacts with the nitrogen, forming AlN on the surface of the powders. If there is any oxygen present, the fracture surfaces react with oxygen to form Al.sub.2 O.sub.3 on the powder particles. The net result of milling NiAl in liquid nitrogen is an arrangement of fine particles of AlN, NiAl and Al.sub.2 O.sub.3 on NiAl powder particle surfaces. Consolidation of the cryomilled NiAl powder via hot extrusion or hot isostatic pressing produces particulate strengthened materials with relatively high volume fractions of AlN. U.S. Pat. No. 4,619,699, to Petkovic-Luton et al., describes a cryomilling process as a means to improve the mechanical properties of high temperature Fe- and Ni-based oxide dispersion strengthened alloys by control of their structure. Cryomilling of NiAl leads to the formation of a discontinuous AlN particle reinforced aluminide that shows good potential as a high creep strength, oxidation resistant material at elevated temperatures. Luton et al., in "Cryomilling of nano-phase dispersion strengthened aluminum," Mat. Res. Soc. Symp. Proc., Vol. 132, 1989, speculate that the high creep strength at elevated temperatures is due to a threshold stress for superplastic flow in the fine grain alloy formed when Al(ON) particles generated during the milling inhibit the grain growth in the aluminum matrix.
In Whittenberger et al., "Preliminary investigation of a NiAl composite prepared by cryomilling," J. Mat. Res., Vol 5, No. 2, Feb 1990, prealloyed Ni-51 (at. %) Al was cryomilled with Y.sub.2 O.sub.3 to improve the high temperature strength resistance of the resultant alloy to form an NiAl -AlN composite alloy wherein the volume fraction of AlN within the NiAl matrix was estimated to be about 10%. Upon compression testing at 1200 and 1400 K, the creep strength was measured to be six times better than that of NiAl. NiAl-AlN composite alloys having volume fractions of up to 13% AlN were reported in Aiken et al., "Reproducibility of NiAl cryomilling," Mech. All. for Struct. App., Proc. 2nd Int. Conf. Struct. Appl. Mech. All., Sept. 20-22 1993 and in Whittenberger et al. "1300 K Compressive Properties of a Reaction Milled NiAl-AlN Composite," J. Mater. Res., Vol. 5, No. 12, Dec. 1990, pp. 2819-2827. Compression testing of the NiAl-AlN alloys at 1100 and 1300 K showed creep strengths superior to those of .gamma.' and oxide dispersion strengthened polycrystalline superalloys and results were considered to be very reproducible.
AlN formed from the reaction of nitrogen with Al during cryomilling has the deleterious effect of depleting Al from the NiAl matrix, thereby lowering the cyclic oxidation resistance of the NiAl base material. Thus, as the AlN volume content of the NiAl-AlN alloy (and related high temperature properties) goes up, the oxidation resistance of the alloy goes down. Efforts to correct this problem include adding about 0.5 wt. % Y.sub.2 O.sub.3 to prealloyed NiAl powder prior to cryomilling. Lowell et al. in "Cyclic oxidation resistance of a reaction milled NiAl-AlN composite," Mat. Res. Soc. Symp. Proc., Vol. 194, 1990, compare the cyclic oxidation resistance of a NiAl-AlN alloy material wherein Y.sub.2 O.sub.3 was added during cryomilling of NiAl with 0.15 atom % Zr. This probably improves the oxide scale adherence, thereby minimizing the Al consumption during cyclic oxidation. The cyclic oxidation limit for the NiAl-AlN was found to be about 1473 K for times greater than 100 hours and 1573 K for times under 100 hours. Additionally, as the Al becomes depleted from the NiAl matrix, the thermal conductivity, an important property of aircraft engines, is also reduced. A further problem associated with Al depletion from the matrix of these alloys includes an increase in density of the material.
In general, a major obstacle for structural applications of NiAl-AlN base materials is its low temperature toughness. The B2-crystal structure of NiAl with limited slip systems is believed to be the cause of lack of low temperature toughness. Several attempts at raising the toughness by alloying additions to modify the crystal structure or opening up additional slip systems in NiAl have shown very limited success.